The instant invention relates to the joining of two contiguously placed sheet metal elements utilizing laser heating of a powdered filler metal introduced into the gap therebetween. It is noted that the term "sheet metal" is generally understood in the art as denoting a metal sheet having a thickness of less than one-eighth of an inch, in contradistinction with "metal plate," which term encompasses metal sheets having a thickness equal to or greater than one-eighth of an inch.
Known methods for joining sheet metal elements include TIC, MIC, and electron beam welding. Unfortunately, the large heat affected zone ("HAZ") generated by these methods induces heat-generated distortion of the sheet metal elements to be joined, often rendering the finished product unserviceable. And, where the application involves a corner joint, for example, heat distortion of one or both of the sheet metal elements tends to dramatically increase the gap therebetween, thereby further complicating the joining process. Moreover, it is crucial that the sheet metal elements not be heated to a molten state during the joining process in order to prevent the heat source from melting them completely away, i.e., burning a hole therethrough. It is further noted that electron beam welding requires a vacuum chamber wherein the entire workpiece is encapsulated, thereby placing severe limitations on its application.
In addition, the prior art teaches autogenous laser welding as a very rapid and economical method of welding relatively thin metallic components with a reduced HAZ. However, autogenous laser welding techniques generally require very tight joints to obtain satisfactory welds. The close tolerances thus required of the surfaces to be joined is impractical or unfeasible in many applications, such as joining two non-coplanar metal sheets at their edges as is required in a "corner joint." Moreover, since the reduced thickness of each sheet greatly diminishes the ability thereof to conduct the extreme heat generated by the welding process away from the area of laser beam impingement thereon, the risk of unacceptable heat distortion or accidental burn-through remains great, particularly with the high power densities often associated with known laser welding techniques.
To accommodate the gaps between the elements to be joined during the laser welding thereof, the prior art teaches the introduction of a filler material into the gaps therebetween. For example, in U.S. Pat. No. 4,634,832 to Martyr, a filler rod is vaporized by a laser beam focussed thereon to form a plasma which heats opposed gap faces while being deposited thereupon to fill in the gap therebetween. However where metal sheets having a thickness of less than one-eighth of an inch are to be joined, rather than metal plates as taught in Martyr, the thickness of the sheets is insufficient to provide for proper containment of the plasma therebetween. A backing strip must therefore be utilized to contain the plasma in the gap during the welding process. However, the use of such a backing strip severely limits the utility of the Martyr method. It is further noted that the patent to Martyr discourages the use of a particulate filler material in place of the rod taught therein, as a backing strip is then required, even when joining the thicker plates.
In U.S. Pat. No. 4,048,459, issued Sept. 13, 1977, to Earle, a gravity feed mechanism is utilized to deliver metal brazing powder between two powdered metal parts placed in close-spaced juxtaposition. The brazing powder, which has a lower melting point than either of the parts in order to prevent the absorption of the brazing powder thereinto, is thereafter brazed by a laser beam impinging directly and solely thereupon, whereby the parts are joined. It is noted, however, that a cavity must be purposefully formed between the parts having a configuration suitable for receiving and containing adequate amounts of filler material to effect the joining of the parts, as the gravity feed mechanism of Earle is incapable of delivering the powder into the minute cracks formed between the edges of elements placed in contiguity with one another. Obviously, such a cavity requires a backing strip or other backing member in order to contain the brazing powder which is introduced thereinto.
In U.S. Pat. No. 4,644,127, issued Feb. 17, 1987, to LaRocca, a laser welding method is taught wherein a flow of powdered material, superheated by passage through a portion of the laser beam, reaches the substrate surface so as to form a "hot blob" thereupon, whereafter it interfuses with the substrate, which was preheated by the direct impingement thereupon of the laser beam. A post-heating of the weld seam by further direct laser beam impingement thereupon is also utilized. Unfortunately, such direct impingement of the laser beam on the sheet metal elements carries with it substantial risk of heat distortion before, during, and after generation of the weld seam, as well as a greater likelihood of burning a hole through one of the sheets due to the extremely high power densities taught by LaRocca.
Similarly, in Japanese Patent No. 58-65,592 to Tokyo Shibaura Electric, Ltd., published Apr. 19, 1983, a second focussed laser beam provides for the post-heating and homogenation of superheated powder filler material deposited by the above "hot blob" technique. While neither the first laser beam utilized to superheat the powder, nor the second post-heating laser beam, impinges directly upon the metal elements to be joined, the post-heating of the pre-deposited powder blobs induces greater dilution of the composition of the metal elements, thereby effecting a change in the physical properties of the final product. Additionally, it is significant that a pre-formed channel is required between the metal elements to be joined since, as in LaRocca, the "hot blob" deposition technique of Tokyo Shibaura Electric, Ltd. is incapable of delivering the superheated powder blobs into the minute cracks formed between the edges of the metal elements placed in contiguity with one another. Moreover, it is noted that, where a gap of varying width is present between the elements to be joined, weld strength is lowered by the inability of the "hot blobs" to adequately penetrate and/or adhere to the gap faces.
It is noted that, in the related field of laser cladding, a laser is used to melt a thin surface layer of a metal substrate and a heterogenous powdered metal deposited thereupon which, upon cooling, generates a coating that is metallurgically bonded with minimum dilution to the metal substrate and has surface characteristics superior to those of the uncoated substrate. In this manner, a low cost metal article may be provided with an exotic coating having superior wear characteristics while achieving significant cost reductions.
For example, in U.S. Pat. No. 4,300,474, issued Nov. 17, 1981, to Livsey, a method and apparatus for laser cladding is taught wherein the laser beam impinges directly upon the substrate, forming a melt pool to which the powdered cladding material, partially heated by passage through the beam, is then added. As in the LaRocca patent, the fringe area of the laser beam in Livsey purposefully impinges upon an area of substrate ahead of the particulate stream to generate a molten pool therefrom, as well as upon the partially solidified metal coating subsequent to particle deposition thereon in order to further homogenize the coating and to improve fusion bonding between the coating and the substrate material. Again, where the substrate comprises a sheet metal element, such areas are susceptible to being burned completely away by the continuing, direct exposure thereof to the laser beam.
It is further noted that the Livsey patent teaches a dynamic powder feed system (hereinafter "DPF") to accommodate complex workpiece geometries for which other methods of cladding material deposition--such as pre-placed powder beds, pre-placed chips, or wire feed--are ill suited. DPF comprises the use of mechanical metering devices in combination with a gas carrier to control and assist the delivery of hardfacing powder to the workpiece.
It is the purpose of the instant invention to provide a method for joining a pair of contiguously-placed sheet metal elements having a gap of varying width in the joint therebetween with minimal heat distortion of the sheet metal elements during such joining.
A further purpose of the instant invention is to provide a method for joining a pair of contiguously placed sheet metal elements which utilizes a beam of coherent electromagnetic radiation while avoiding the direct impingement of the beam on either of the sheet metal elements.
It is also the purpose of the instant invention to improve upon existing laser welding techniques to permit the welding of sheet metal elements having gaps of varying width therebetween, without the use of a backing strip, or the need for special filler materials characteristically having lower melting temperatures than the material of the sheet metal elements.